1. Field of the Invention
The present invention relates generally to implantable cardioverter defibrillator (ICD) system and, more specifically, to a method and apparatus for utilizing capacitors systems having effective capacitances of less than about 60 .mu.F, i.e., short tau capacitors, in an ICD system.
2. BACKGROUND OF THE INVENTION
The incidence of heart disease in the United States is significant with approximately one out of every two deaths is attributable to heart disease. One of the leading complications secondary to heart disease is cardiac arrythmia resulting in sudden cardiac death. Because of the high prevalence of sudden cardiac death due to cardiac arrythmia there is a demonstrated need for an implantable cardioverter defibrillator (ICD) system. To be useful, an ICD systems must be self-contained, complete, and capable of effective repetitious function autonomous from the outside world. A direct corollary of these requirements is that the system be small enough to be implanted. The size limitations imposed by an implantable device generally have prevented the technology and techniques applicable to external defibrillator systems from being applied to ICD systems. Even with the development of novel technology and techniques for generating defibrillation pulses or countershocks in an implantable device, present day ICD systems are still sufficiently large so as to require implantation within the abdomen or abdominal wall of a patient.
A more ideal size for an ICD system would be the size that implantable cardiac pacemakers have achieved, such that the device is capable of being implanted in the subcutaneous space just inferior to either clavicle. Unfortunately, pacemakers are capable of smaller sizes quite simply because their power requirements are significantly less than the power requirements of ICD systems. While pacemakers have energy output requirements that are in the microjoule output range, ICD systems, to be effective, must be capable of delivering repeated defibrillation countershocks above at least about 15 joules for each countershock. In order to achieve such high energy outputs, existing ICD systems have utilized larger batteries, as well as large high voltage capacitors to generate the required high energy defibrillation countershock. Consequently, one of the major challenges in reducing the size of an ICD system is how to decrease the effective size limits of both the batteries and capacitors that are used by the ICD system.
Presently, there are three different types of ICD systems which have received device approval from the Federal Drug Administration, the PCD.TM. device, available from Medtronic, Inc., of Minneapolis, Minn., the Cadence.RTM. device, available from Ventritex, Inc., of Mountain View, Calif., and the Ventak-P.RTM. device, available from Cardiac Pacemakers, Inc., of St. Paul, Minn. The primary components of all existing ICD systems include an automatic monitoring and detection mechanism, a capacitor system, a battery system and control circuitry for detecting a ventricular arrhythmia and controlling delivery of a high-voltage capacitive-discharge electrical countershock in response by charging and then discharging the capacitor system. To achieve successful defibrillation, the ICD system must deliver a high voltage electrical countershock with an initial voltage of greater than about 500 to 600 volts.
The existing ICD systems are all capable of delivering a maximum countershock of up to 700 to 750 volts having a total energy of between 31 to 44 joules. At the time an ICD system is implanted in a patient, the attending physician will empirically determine a minimum defibrillation threshold for the patient, and will program the charging voltages for the countershocks to be delivered as part of a therapy regimen within the range of maximum voltages allowed by the device. In addition, the attending physician can also typically program when the electrical countershock is to be truncated by programming either the duration of the countershock in a range from about 6 to 9 milliseconds. Alternatively, the duration can be altered by programming the initial discharge voltage of the ICD system and allowing the tilt of the ICD system to establish the point at which the countershock will be truncated. The tilt of an ICD system is defined as the percentage decline in the output voltage from the charging voltage to the voltage at the time the discharge is truncated. For existing ICD systems, the tilt is typically set at about 65%.
To understand why the battery and capacitor systems of existing ICD systems are designed as they are, it is helpful to examine the relationships and limitations of the various components of the ICD system that are used to produce an effective electrical countershock. To begin with, the capacitance of an ICD system is proportional to the applied charging voltage and the stored energy. The relationship is given by: EQU E=0.5(C*V.sup.2) (1)
where E, in joules, is the amount of energy stored in the capacitor, C is capacitance and V is charging voltage. Transforming this equation yields: EQU C=2(E/V.sup.2) (2)
Consequently, if the maximum stored energy and the maximum charging voltage of an ICD system are known, there will be a minimum effective capacitance of the ICD system that is necessary to achieve the chosen values for maximum stored energy and charging voltage. In existing ICD systems, which have maximum stored energy values of 35 to 40 joules and maximum charging voltages of 750 volts, the minimum effective capacitance of the ICD system is approximately 120 to 140 .mu.F. In existing ICD systems, the maximum stored energy has been selected so as to provide a defibrillation threshold safey margin, and the maximum charging voltage is determined by the choise of a pair of electrolytic capacitors as the capacitor system. Thus, the effective capacitance of existing ICD systems is a value that cannot be arbitrarily chosen, but is instead dictated by other factors in the design of the ICD system.
In addition to affecting the maximum stored energy for an ICD system, the effective capacitance of an ICD system also impacts the duration of the coutnershocks whih are delivered by the ICD system. The rate of discharge of a capacitor system is given by: EQU .tau.=R*C (3)
where R is the average myocardial tissue resistance and .tau. is a time constant tau of the ICD system. If the capacitance C is 140 .mu.F and the resistance R across the myocardial muscle is 50 ohms, then, according to Eq. (3), .tau. equals 7 milliseconds.
The time constant .tau. describes the natural exponential decay of the capacitive discharge from an initial discharge voltage V.sub.i to a final discharge voltage V.sub.f at the truncation such that: EQU V.sub.f =V.sub.i *e.sup.-d/ .tau. (4)
where d is the duration of the pulse in terms of the duration of a monophasic waveform or the end of the first phase of a biphasic waveform. When the duration d of the discharge output is truncated at about one .tau., as is the case for all existing ICD systems, the final discharge voltage V.sub.f will be about 38.8% of the initial discharge voltage V.sub.i.
Because energy is a function of the square of the charging voltage, most of the stored energy is delivered to the heart by a countershock that has a duration of about one .tau.. For the capacitance values and charging voltages of existing ICD systems, the percentage of stored energy that is actually discharged from the capacitor through the myocardium in about one .tau. is about 80 to 90% of the total stored energy. Assuming that the initial charging voltage V.sub.i is 750 volts and that C is 140 .mu.F, for example, then the total stored energy as defined by Eq. (1) is 39.3 joules. In this case, if the electrical countershock is truncated at 7 milliseconds, then the final discharge voltage V.sub.f is 291 volts from Eq. (4). Using the final discharge voltage Vf in Eq. (1), it can be seen that 5.92 joules of energy will remain in the capacitor system. In this example, a total of 33.3 joules of energy will be delivered to the myocardium, about 85% of the total stored energy.
It can be seen from this example that truncating the discharge of existing ICD systems at a duration less than one time constant tau significantly decreases the proportion of stored energy which is actually delivered to the heart and would have the undesired effect of actually decreasing the effectiveness of the ICD system without providing any decrease in the overall size of the device.
In a co-pending application entitled IMPLANTABLE CARDIOVERTER DEFIBRILLATOR SYSTEM HAVING A SMALLER DISPLACEMENT VOLUME, Ser. No. 08/033,632, filed on Mar. 15, 1993, it is taught that the overall size of an ICD system can be decreased by decreasing the effective capacitance of the ICD system below about 120 .mu.F. Because the size of a capacitor system is proportional to its capacitance value, a decrease in the capacitance value decreases the size of the capacitor system, and, hence, the overall size of the ICD system.
Even though the capacitance C is descreased, and therefor the energy E as defined by Eq. (1) is decreased, the ICD system of the co-pending application actually increases the overall effectiveness of the device due to the fact that when the capacitance C of the ICD system is less than 120 .mu.F, it is more closely matched to the natural capacitance of the heart than the capacitances of existing ICD systems. As a result, the physiologically effective current delivered by a capacitor system having an effective capacitance of less than 120 .mu.F is actually greater than that of existing ICD systems. In other words, an ICD system with a smaller capacitance is more effective than an ICD system with a larger capacitance. For a more detailed explanation of this phenomenon, reference is made to the co-pending application identified above.
While signficant decreases in the overall size of an ICD system can be obtained by decreasing the effective capacitance C of the ICD system, there is a point below which decreasing the effective capacitance of an ICD system begins to have a negative impact on the effectiveness of the defibrillation countershock. To understand why this happens, it is necesary to examine the average current of the delivered countershock.
Imagine delivering a 10 volt pulse through a resistance of 50 ohms for 10 seconds. Using Ohm's law: EQU I=V/R (5)
where I is current, V is voltage, and R is myocardial resistance, then the current is 0.2 amperes. The energy of the pulse is given by: EQU E=V*I*d (6)
where d is the pulse duration, which works out to be 10 volts.times.0.2 amperes.times.10 seconds which equals 20 joules of energy delivered.
Unfortunately, 20 joules of energy delivered in this fashion will never defibrillate the heart because the average current flowing through the myocardium is insufficient. Experiments dating back to the 1890's have demonstrated that there is a minimum threshold average current that is necessary to consistently achieve successful defibrillation countershocks. In these experiments, this minimum threshold was defined as the rheobase current for a particular cell in terms of the minimum average current needed to effect a successful defibrillation countershock if the current was applied for an infinite duration. This rheobase value was derived from strength-duration curves from experimental data and is the asymptotic value projected for the experimental data as the duration approached infinity.
From these experimental strength-duration curves, a value known as the chronaxie was also defined as that duration of time that requires a minimum average current threshold that is twice the rheobase to effect defibrillation. Numerous experimental studies on animal hearts have shown the defibrillation chronaxie range to be approximately 2 to 4 milliseconds. The rheobase has been shown to be approximately 5 amperes. There is no experimental or clinical reason to expect the human heart to have rheobase and chronaxie values outside this range.
The minimum threshold current needed to defibrillate a heart with a very wide pulse would be very near the rheobase of 5 amperes. Using Ohm's law and the accepted value of 50 ohms as the resistance between discharge electrodes, then a 5 amp current through 50 ohm will require 250 volts. A very wide defibrillation pulse duration, for example 40 milliseconds, will require at least 50 joules of energy to be effective using Eq. (6). Using a defibrillation chronaxie duration of 3 milliseconds, then the minimum threshold average current will be 10 amperes, or twice the rheobase. This will require a charge of 500 volts, but will effect defibrillation with only 15 joules total energy delivered in a duration of about 3 milliseconds.
For time periods shorter than the chronaxie, the minimum energy requirement begins to increase again, for example a 1 millisecond pulse will require a minimum of 20 joules to reach the defibrillation threshold. The usefulness of the chronaxie becomes apparent because it identifies the duration of discharge most efficient in terms of the energy required to successfully carry out a defibrillation. For safety concerns, all ICD systems should be able to deliver a defibrillation countershock that exceeds 500 volts, 10 amperes average current, and a 3 millisecond duration in order to ensure that the countershocks which are delivered will succeed in countershocking a fibrillating heart.
The average current of 10 amperes over 3 milliseconds is based on a rectangular waveshape. In external defibrillating devices, where system size is not a concern, the systems have been developed to deliver generally rectangular shaped waveform. On the other hand, research has shown that the exponential decay curves of a capacitive-discharge is also effective in consistently defibrillating the heart; however, the calculation for the average discharge current is not as straight forward as for a rectangular waveshape. An average discharge current for an exponential decay capacitive discharge can be calculated from the following equation: EQU Iave=C*(V.sub.i -V.sub.f)/d (7)
where C is capacitance, V.sub.i is the initial charging voltage, V.sub.f is the final voltage at time d when the discharge is truncated.
Using Eq. (7), it is possible to determine a minimum effective capacitance that will deliver an lave equal to 10 amperes when the duration d equals the chronaxie at 3 milliseconds, and (V.sub.i -V.sub.f) equals 500 volts. Solving Eq. (7) under these conditions yields a minimum capacitance C equal to at least 60 .mu.F in order to produce a defibrillation countershock that will meet the minimum average current, duration and voltage conditions established above.
The problem with solving Eq. (7) in this manner is that the term (V.sub.i -V.sub.f) is not limited to any particular V.sub.i. Due to the charging voltage restrictions for the pair of electrolytic capacitors used in existing ICD systems that limit V.sub.i to 750 volts, it is necessary to resolve Eq. (7) so that this limitation may be factored into the equation. Reworking Eq. (4), it is possible to define the term (V.sub.i -V.sub.f) as related to the .tau. of the system by the equation: EQU V.sub.i -V.sub.f =V.sub.i *(1-e.sup.-d/.tau.) (8)
Substituting Eq. (8) for the term (V.sub.i -V.sub.f) in Eq. (7) yields a solution to the minimum theoretical capacitance necessary for a pair of electrolytic capacitors charged to a maximum voltage of 750 volts in order to deliver a defibrillation countershock with Iave equal to 10 amperes when the duration d equals the chronaxie at 3 milliseconds, and (V.sub.i -V.sub.f) equals 500 volts: EQU C=(Iave*d)/(V.sub.i *(1-e.sup.-d/.tau.)) (9)
Solving Eq. (9) for C under these conditions gives a value of 63 .mu.F as the minimum theoretical capacitance for an ICD system that uses a pair of electrolytic capacitors. When this theoretical capacitance of 63 .mu.F and a maximum charging voltage of 750 volts are used in Eq. (1), the maximum stored energy of such a theoretical ICD system would be about 17.7 joules, of which about 85% can be delivered in one time constant tau, or about 15 joules. This value corresponds with the results of experimental data that teach that the minimum efficient amount of energy needed for reliable defibrillation is 15 joules delivered over 3 milliseconds.
Even thought the solution to Eq. (9) suggests that a smaller effective capacitance could be used, existing ICD systems have been designed for a 100% stored energy safety factor such that they can provide for a maximum stored energy of at least about 35 joules, as opposed to the 17 joules stored for the theoretical minimum capacitance. This practice has dictated the use of two electrolytic capacitors with an effective capacitance in the range of at least about 140 .mu.F. The doubling of the capacitance also doubles the time constant for the system, which in turn doubles the discharge duration. As a result, all of the ICD systems in use today have followed these guidelines and use a minimum duration of approximately 6 milliseconds, which is twice the expected defibrillation chronaxie value of 3 milliseconds, and provide for a maximum stored energy of at least 35 joules.
Even though it is known that smaller capacitances have smaller volumes, the minimum theoretical capacitance of about 60 .mu.F appears to be a practical lower limit on the size and effective capacitance of a capacitor system for use in an ICD system. Below about 60 .mu.F, the duration of the countershock discharge truncated at one time constant .tau. starts to fall below the defibrillation chronaxie value of 3 milliseconds. In other words, capacitors below this value have time constants which are too short for effective defibrillation.
Although existing ICD systems have proven effective in providing defibrillation therapy, the existing devices are larger than desirable. While it is possible to decrease the size of an ICD system by decreasing the effective capacitance and, hence, the size of the capacitor system, there is a minimum theoretical capacitance below which the effectiveness of the defibrillation countershock significantly decreases. Consequently, it would be desirable to provide for a method and apparatus which would allow an ICD system to utilize a smaller capacitor system that has an effective capacitance below the minimum theoretical capacitance without significantly decreasing the effectiveness of the defibrillation countershock generated by such a smaller capacitor system.